1. Technical Field
The present invention relates to image display devices and image display methods thereof, and image display programs, in particular, to a hold type image display system.
2. Description of Related Art
A liquid crystal display (LCD) is being widely used since it is thin and does not occupy as much installation area compared to a CRT (Cathode Ray Tube) display. However, in the hold type display device such as LCD, an image is continued through a frame period as opposed to an impulse type display device such as CRT display, and thus a moving image tends to become unclear.
In the case of the impulse type display device, an image is displayed as a pulse at an early stage in the frame and a black display is displayed until the next frame, and thus the afterimage occurrence is adjusted not to be recognized by the user's eyes. In the case of the hold type display device, on the other hand, the image is held and displayed as a still image within the frame period, and the moving image is displayed by switching the screen for every frame, and thus the still image is seamlessly switched from one frame to another, whereby the user recognizes the previous frame image as an afterimage, senses a double image in which the shifted images are overlapped and recognizes a moving image blur. In order to reduce the moving image blur in the hold type display device, a hold type display device for pseudo-driving in an impulse type display is disclosed in Japanese Patent Publication No. 3385530 (patent document 1) and Japanese Laid-Open Patent Publication No. 2001-166280 (patent document 2).
Since the liquid crystal generally deteriorates when a direct current component is applied to the liquid crystal over a long period of time in the liquid crystal display device, a frame inversion drive of inverting the voltage polarity applied to the pixel for every frame is performed.
However, in most liquid crystal display devices pseudo-driven in the impulse type display, a black signal and a video signal are alternately written to each pixel at a constant cycle, and thus only the black signal is written in one polarity and only the video signal is written in the other polarity if the cycle of inversion drive and the cycle of black insertion coincide, whereby the DC component is applied to the liquid crystal panel causing burning of the liquid crystal panel and degradation in lifetime.
In the method disclosed in patent document 1, in order to improve such drawbacks, the polarity is inverted at a cycle of double the cycle of black insertion in the black insertion drive of alternately repeating the black signal and the video signal for every frame. In the case of the black insertion drive repeated for every frame, to which pixel of the display screen each black signal is used can be easily defined, and thus burning of the liquid crystal panel and degradation in lifetime can be avoided by forming a sequence with the switching of the frames of the video signal as a base point.
However, in the display device of patent document 2 configured as shown in FIG. 18, since the black insertion drive of inserting the black image at a specific proportion within one frame is performed, to which pixel of the display screen the black signal is used differs depending on a black insertion rate, and the black signal is positioned in the middle of the display screen at the switching of the frames of the video signal, whereby it is difficult to form a sequence of inversion with the switching of the frames of the video signal as the base point.
Patent document 1 discloses a method of inverting the polarity for every two outputs of the video signal and the black signal following thereto, where if such method is used in the device of patent document 2, the order of inversion of the black signal and the video signal changes at the middle of the screen, as shown in FIG. 19, whereby luminance difference and burning might occur at the display with a line as a boundary at which the polarity inversion switches according to variation in field through of the panel surface and variation in positive and negative of the application voltage (see FIG. 20).
It is an exemplary object of the present invention to provide a hold type image display system capable of flexibly responding to switching of black insertion rate, and capable of preventing display luminance difference and burning at the line at which the polarity inversion switches as a boundary.